The invention relates to a device for inserting a sensor into the exhaust conduits of an internal combustion engine, and in particular a sensor of the electrochemical type collecting concentrations of species in the exhaust gases.
Such sensors are still called probes .lambda.. Their voltage response curves show a sudden swing at the stoichiometry of the fuel-air mixture whose relative concentrations in the exhaust gases it is desired to measure. The analysis is effected by taking samples of exhaust gases which alone are admitted inside the sensor (where they are brought to a thermodynamic balance), in accordance with the so-called "test sampling" technique. To reach this result means for limiting the gaseous exchange are usually disposed upstream of the sensor. The electric signals delivered by such a sensor are then used by a regulation system for modifying, for example, the air-fuel proportion admitted into the cylinders of a combustion engine. This regulation method is well-known in the automobile electronics field.
These sensors have been constructed with different approaches: in accordance with a first earlier approach, the active element or measuring cell, generally of the concentration-stack type, has a so-called "glove finger" structure; in accordance with a second approach, the active part of the sensor is formed by thin or thick-film depositing techniques used in the manufacture of semiconductor circuits, and presents a flat structure.
In all cases, the measuring cell must be inserted into the exhaust conduits of a combustion engine in which it is desired to analyze the composition of the exhaust gases. For this, according to the prior art, the measuring cell is inserted into a case having the general structure of a sparking plug. This case must provide, on the one hand, an absolute seal between a first enclosure, or measuring enclosure, in contact with the gases to be analyzed and a second so-called reference enclosure and, on the other hand, provide the electrical connections between the measuring cell and external circuits using the electrical signals delivered. These different requirements lead to a complex and relatively costly structure. Furthermore, insofar as the second approach is concerned, the flat geometry of the measuring cell must be adapted to the cylindrical geometry of the case. Finally, some more recent sensors comprise more than two electrical connections, but the structures of cases of the sparking-plug type do not lend themselves very well to bringing out more than two connections.
In addition to the difficulties which have just been pointed out, allowance must also be made for the need for transforming existing exhaust conduits as to be able to insert therein by screwing the case containing the measuring cell. Finally, since the type of sensor which has just been described may present a high output impedance, the insulators must be of a good quality, and this at a high temperature, which is difficult to obtain.